The present invention relates in general to ultrafiltration membranes, and in particular to a new and useful polyethersulfone membrane and method for making such membrane by graft polymerization of particular vinyl monomers in the presence of UV radiation and a filter so that the membrane exhibits high solute retention, high permeability, and low fouling characteristics.
Ultrafiltration membranes have found widespread use in the food and biotechnology industries. Ultrafiltration (UF) has been applied in the processing of normal and transgenic milk, cheese and eggs, whey and potato protein recovery, the clarification of juices and wine, the recovery of proteins from animal blood, and the purification of water. UF is also used in the biotechnology industry for the recovery of biological products through such steps as cell broth clarification, cell harvesting, concentration or diafiltration of protein solutions prior to separation, and final concentration.
A major obstacle in the incorporation of membrane processes into industrial operations is the problem of flux decline due to fouling during the ultrafiltration of biological products such as proteins. Fouling not only decreases membrane permeability which reduces productivity due to longer filtration times, but also shortens membrane life due to the harsh chemicals necessary for cleaning. Furthermore, fouling can alter membrane selectivity and lead to significant product loss through denaturation.
While the exact mechanism of flux loss during protein filtration is not clear, the general consensus seems to be that the main causes are: osmotic back-pressure from concentration polarization, adsorption or deposition of proteins on the surface or in the pores, and compaction or consolidation of the adsorbed protein layer on upstream side of the membrane. Fouling is the reversible and irreversible adsorption and deposition of proteins and protein aggregates on the membrane surface and in the pores. This causes narrowing or plugging of membrane pores, which results in decreased membrane permeability. Irreversible fouling causes flux loss that is recovered only through the use of harsh detergents and/or chemicals. Flux loss caused by reversible protein fouling, however, is temporary for the protein can easily be removed by rinsing the membrane with water. Furthermore, after some time and under the right operating and solution conditions, further flux loss can occur because the adsorbed protein layer(s) can consolidate or compact into a more dense, higher flux-resistant layer.
To solve to reduce and remove flux loss, UV-assisted grafting of a monomer onto a membrane has been implemented. Grafting consists of attaching a smaller chemical unit to a main molecular chain. In the past, photoinitiators were used to initiate free radical polymerization at the membrane surface. However, the preferred method of attachment is by UV irradiation rather than plasma or chemical means, which has the advantages of simplicity and short reaction time. UV radiation is generally considered to have a wavelength range from 100 to 450 nm. UV irradiation can crosslink polymer chains and cleave polymer bonds, forming functional groups such as hydroxyls, carbonyls, or carboxylic acids on the membrane surface. Chemical bonds in the membrane polymer are cleaved directly. Free radical sites can be formed on the membrane surface through the cleavage of polymer bonds. When vinyl monomers are present, free radical graft polymerization occurs at these sites, forming polymer chains that are covalently bonded to the surface.
U.S. Pat. No. 5,468,390, and articles in Journal of Membrane Science 105(1995), p. 237-247 and Journal of Membrane Science, 105 (1995, p. 249-259), disclose modified aryl polysulfone membranes having a hydrophilic vinyl monomer chemically grafted to their pore wall surfaces. An unmodified membrane is contacted with a solution of the monomer and is exposed to ultraviolet light to effect photochemical grafting in the absence of a sensitizer or a free radical initiator. The monomers utilized function to render only the polysulfone membrane pore wall surface hydrophilic. The remaining portion of the membrane solid matrix comprises unmodified polysulfone. These surface-modified membranes are not rewettable after they have been dried and, if dried, lose significant permeability. Therefore it is necessary to maintain the membrane surfaces wet prior to use.
Japanese Pat. No. JP-A-2-59029, published Feb. 28, 1990, discloses a process for modifying a polysulfone porous membrane on its pore wall surface only with a polymerizable monomer by immersing the membrane in the monomer solution and irradiating the solution with ultraviolet light. The process is conducted under conditions such that any solvent used in the process does not dissolve the polysulfone membrane. As a result of the process, only the pore wall surface of the porous membrane is modified to render it hydrophilic when hydrophilic polymerizable monomers are utilized in the process. U.S. Pat. No. 5,468,390 teaches a method for photochemically modifying polysulfone UF membranes using ultraviolet (UV) assisted graft polymerization of hydrophilic vinyl monomers to reduce flux loss during the filtration of protein solutions. In particular, irradiation at 254 nm was used to create the radical sites necessary for the polymerization of the monomers to the membrane. The disclosed method consists of immersing the polysulfone membrane in a solution of hydrophilic vinyl monomer, and then irradiating the membrane at a wavelength of 254 nm.
However, recent testing has shown that even though UV-assisted graft polymerization achieved by an immersion technique successfully decreased fouling of membrane by imparting hydrophilicity to the surface, the membrane permeability was found to decrease sharply after modification due to blockage of the pores by the grafted polymer chains, caused by high chain density and long grafted chains. A necessary balance is sought between sufficient surface hydrophilicity for low fouling and higher membrane permeability. Furthermore, polysulfone membranes are not very photoactive (e.g., bonds are less easily broken), and therefore, are more difficult to modify. Accordingly, a modified membrane is desired that has high photoactivity, high graft density of short chains on its surface, exhibits low protein fouling, and maintains a greater fraction of the original membrane permeability after modification.
Furthermore, the immersion technique used in the prior art requires a large amount of monomer and is less adaptable to continuous processes on an industrial scale. Also, the immersion technique used in the prior art results in a high UV absorbance of monomer or shielding by the monomer solution, and a considerable amount of UV light does not reach the membrane. When UV light passes through to the membrane using the dipping technique, pore enlargement and a loss in protein rejection is observed because the UV light is not absorbed by the monomer solution as in the immersion technique. UV light at a wavelength of 254 nm has high energy capable of enlarging and damaging the pores, and thereby rendering the membrane useless. Therefore, a technique is required that will use less monomer and will allow a higher intensity, but lower energy UV light to reach the membrane, particularly because high intensity lower energy UV light optimizes the surface chain density and chain length for maximizing membrane permeability and retention properties, while reducing non-fouling characteristics.
Polyethersulfone is an optimal membrane material because it is widely used as a membrane in the biotechnology industry, it is relatively hydrophobic, and is intrinsically photoactive. Addition of a chain transfer agent facilitates termination of the monomer at various points to reduce the size of the grafted monomer chain. Furthermore, when modification is conducted with a dip technique rather than an immersion technique, a lower energy UV light in the range of 280 to 300 nm is desirable to prevent damage to the membrane because significantly more UV light reaches the membrane in the dipping technique. A filter is needed to assist in shielding out high energy UV light, and hence reducing pore enlargement. The altered dip modification technique using a liquid or solid filter will create modified membranes with lower protein fouling and reduced pore enlargement.